FIGS. 4(a) to 4(f) are cross-sectional views showing a prior art method for producing an infrared detector. As shown in FIG. 4(a), p-type CdHgTe is epitaxially grown on a CdTe substrate 1 by metal organic chemical vapor deposition (MOCVD) or liquid phase epitaxy (LPE) to form a p-type CdHgTe layer 2 about 15 to 20 microns thick. An insulating film 4 comprising ZnS is evaporated and deposited on CdHgTe layer 2 to a thickness of about 2 microns and a diffusion mask pattern shown in FIG. 4(a) is formed by photolithography and etching.
As shown in FIG. 4(b), an In layer 7 is deposited on the insulating film 4 by evaporation. In layer 7 is a diffusion source of dopant impurities producing n-type conductivity in the CdHgTe layer 2.
As shown in FIG. 4(c), indium is thermally diffused from the In layer 7 into the p-type CdHgTe layer 2, forming n-type CdHgTe regions 3. Each region in the p-type CdHgTe layer 2 including the n-type CdHgTe region 3 functions as a pixel.
After the diffusion of indium, as shown in FIG. 4(d), the In layer 7 and the insulating film 4 are removed by etching. An insulating film 10 comprising ZnS is evaporated and deposited on the entire surface of the wafer and an opening 11 and openings 12 are formed at regions where a p-side electrode and n-side electrodes will be formed in a later process, respectively, using photolithography and etching, as shown in FIG. 4(e). Then, a p-side electrode 6 and n-side electrodes 5 are formed in the opening 11 and the openings 12, respectively, completing the infrared detector shown in FIG. 4(f).
In the infrared detector shown in FIG. 4(f), infrared light which is incident on the rear surface of the wafer, i.e., the rear surface of substrate 1, travels through the substrate 1 to reach the pixel region in the p-type CdHgTe layer 2, the n-type CdHgTe region 3, and the pn junction 8 and is absorbed by the n-type CdHgTe region 3, thereby producing electron and hole pairs. Then, the electron-hole pairs are separated into electrons and holes by the pn junction 8 at the boundary between the p-type CdHgTe layer 2 and the n-type CdHgTe region 3 whereby an electromotive force is generated between the p-type CdHgTe layer 2 and the n-type CdHgTe region 3. By detecting this electromotive force, the intensity of the incident infrared light is determined.
In order to enhance the sensitivity of such an infrared detector, the crystalline structure of the semiconductor layer functioning as a pixel must be as uniform as possible for swift movement of electrons and holes. Therefore, when producing pixel regions by diffusing dopant impurities, it is desired to form regions having few crystal defects by using as high a temperature as possible. On the other hand, in order to improve the performance of the infrared detector, it is necessary to improve resolution as well as sensitivity. More specifically, in the infrared detector shown in FIG. 4(f), it is necessary to narrow the width of the non-pixel region WP where only the p-type CdHgTe layer 2 exists. Thereby, the pixel regions including the n-type CdHgTe regions 3 are densely produced.
In the prior art production method shown in FIGS. 4(a)-4(f), a mask is used to make the width of the non-pixel region 2a adjacent to the respective pixel regions narrow, i.e., to make the interval between the centers of the adjacent pixel regions less than 50 microns, for a high density of pixels. Indium is diffused from the In layer 7 by heating these layers and regions to a high temperature (for example, 200.degree. C. or more) to form the CdHgTe regions 3. The diffusion not only advances in the depth direction but also advances in the transverse direction, parallel to the substrate surface, as shown in FIGS. 5(a) to 5(c), whereby the p-type CdHgTe layer 2 beneath the insulating film 4 is also converted to n-type. Therefore, the non-pixel region 2a occupied by only the p-type CdHgTe layer 2 is narrower than required and the conductivity type of the region beneath the mask 4 is sometimes completely converted to n-type, as shown in FIG. 5(d). In the infrared detector thus produced, resolution is not improved but, on the contrary, is reduced.